Powder metallurgy as a perfect technique for preparation of Cu–TiO2 composite by identifying their microstructure and optical properties

Powder metallurgy (PM) is a technique that involves the manufacturing of metal powders and their consolidation into finished products or components. This process involves the mixing of metal powders with other materials such as ceramics or polymers, followed by the application of heat and pressure to produce a solid, dense material. The use of PM has several advantages over traditional manufacturing techniques, including the ability to create complex shapes and the production of materials with improved properties. Cu–TiO2 composite materials are of great interest due to their unique properties, such as high electrical conductivity, improved mechanical strength, and enhanced catalytic activity. The synthesis of Cu–TiO2 composites using the PM technique has been gaining popularity in recent years due to its simplicity, cost-effectiveness, and ability to produce materials with excellent homogeneity. The novelty of using the PM technique for the preparation of Cu–TiO2 composite lies in the fact that it enables the production of materials with controlled microstructures and optical properties. The microstructure of the composite can be fine-tuned by controlling the particle size and distribution of the starting powders, as well as the processing parameters such as temperature, pressure, and sintering time. The optical properties of the composite can also be tailored by adjusting the size and distribution of the TiO2 particles, which can be used to control the absorption and scattering of light. This makes Cu–TiO2 composites particularly useful for applications such as photocatalysis and solar energy conversion. In summary, the use of Powder Metallurgy for the preparation of Cu–TiO2 composite is a novel and effective technique for producing materials with controlled microstructures and optical properties. The unique properties of Cu–TiO2 composites make them attractive for a wide range of applications in various fields, including energy, catalysis, and electronics.

Preparation of Cu-TiO 2 photocatalysts. Cu powder mixtures contain 10, 20, 30 & 40 wt.% TiO 2 which has been mixed using Zirconia ceramic balls is used in the mechanical mixing process, in which if stainless steel balls are used some contaminations with iron can to takes place. But Zirconia balls are inert for any reaction and so hard. The ball mill used in the preparation process in planetary four vails ball mill machine. The copper powder used in this work is atomized semispherical copper. A ball mill technique for 24 h until a homogeneous mixture is obtained. Table 1 summarizes the specifications of the matrix and reinforcements employed for this study. This paper applied the powder metallurgy method to produce the recommended hybrid Cu-TiO 2 nanocomposite. First, the composite powders of Cu and TiO 2 were weighed concerning the required fractions using a sensitive electronic balance of 0.1 mg accuracy level. Then, the weighted composite powders were mixed in a stainless-steel vial and protected from oxidation using pure argon, with a 20:1 steel ball-to-powder ratio (BPR), a ball diameter of 5 mm, and a rotational speed of 250 rpm. Stearic acid (1.5 wt.%) was used as a processcontrolling agent (PCA). Figure 1 shows the composition and nomenclature of prepared specimens.
Characterization of Cu-TiO 2 composite for the photocatalysts. The investigations on the microstructural characteristics of the composite powders were achieved using scanning electron microscopy and energy-dispersive spectroscopy (SEM/EDS). The main goal of using such analysis techniques is to identify the uniform dispersal of the reinforcement materials in the matrix, the composites microstructure, and the com-Scientific Reports | (2023) 13:7034 | https://doi.org/10.1038/s41598-023-33999-y www.nature.com/scientificreports/ posites phases. According to that, this paper also applied the X-ray diffraction (XRD) technique to identify the phases of the mixed powders using a diffractometer with Cu K-alpha radiation and operated at 40 kV. The samples are examined using IR spectroscopy to investigate the absorption band spectra. Also, the antireflection properties were studied. On the other hand, the electrical and thermal conductivities were evaluated using the PCE-COM20 electric resistivity instrument. Thermal conductivity can be calculated using Eq. (1) 24 .
where K refers to the thermal conductivity in W/m. K, L is the Lorentz number (for composites L = 2.45 × 10 -8 W Ω K −2 ), T denotes the absolute temperature in K, and finally, σ is the electrical conductivity in Ω −1 m −1 .
Conductivity of the composite powder. Samples for the electrical conductivity measurements were produced by compacting the milled powders at a pressure of 0.37 GPa at 90C. The diameter of the samples was 10 mm with a height of 6 mm. The electrical resistivity of the compacted samples was measured at room temperature (50% relative humidity) between gold electrodes with an alternating current method at a frequency of 1 kHz.

Results and discussion
XRD analysis. The main task of XRD is to determine and examine the phase composition & phase structure of Cu-TiO 2 crystallinity. XRD is a non-destructive method that is widely used to characterize crystalline material. The structure, phase, crystallinity, and materials' sizes have been shown by using XRD analysis. Scherrer equations are used to calculate the crystal size of the material 25 .
(1) K = LTσ , www.nature.com/scientificreports/ where d is crystallite size, β is the full width of half maximum, θ is the diffraction angle, and λ is the X-ray wavelength of X-ray radiation 26 . The diffraction patterns for various concentrations of TiO 2 doped in Cu are shown in Fig. 2. Two phases of tetragonal TiO 2 are seen when doped with 40% TiO 2 ; one of these phases is anatase TiO 2 (the peak with the highest intensity), and the other phase is rutile TiO 2 (the peak with the lowest intensity that is adjacent to the peak with the highest intensity, which represents a very small amount of rutile TiO 2 ) (the as-resaved TiO 2 powder has both the anatase & rutile phases). High photocatalytic activity may be attributed to this very modest quantity, which functions in the anatase phase as a structural defect or impurity. Following a reduction in the percentage of TiO 2 , only anatase diffraction peaks were seen in samples containing various amounts of TiO 2 . It is also possible to notice that the majority of the 2 peak locations of the primary diffraction pattern do not move, having identical values of pure Cu, except for variations in the intensities of these peaks. This is something that can be seen in all the samples (i.e. intensity decreases as TiO 2 increases). Because the radii of Ti 4+ ions are too large to replace Cu + ions in the Cu matrix, the addition of TiO 2 did not result in any significant modifications to the crystallinity of the material. Aside from the peaks associated with copper and titanium dioxide, there are no other peaks that relate to any novel compounds or phases. This is a reference to the lack of a reaction between copper and titanium dioxide.
There have been no observations of any intermetallic peaks between copper and titanium dioxide, and this is a direct result of the well-regulated milling done by the machine. The structure of the copper metal is known as FCC (Face Centered Cubic), and its atomic radius is 128 pm. In contrast, the structure of titanium is known as HCP (Hexagonal Close Packed), and its atomic radius is 147 pm. Cu with its smaller atomic radius can replace Ti atoms or interstitial incorporated in the Ti crystals, which is caused by the interaction between the crystals of Cu and Ti, which is caused by the mixing of Cu and TiO 2 using mechanical milling for a long time with a high rotation speed. This happens because of the interaction between the crystals of Cu and Ti. Because of this, there are certain shifts in the crystallite structure, which is a sign of successful mixing between the Cu matrix and TiO 2 as a reinforcement. To a greater extent, the interaction between Cu and TiO 2 may be seen to have taken place as a result of the examination of the crystal structure if one uses appropriate settings for the mechanical milling process. To determine the size of the crystallites, the full width at half maximum (FWHM) of the diffraction peaks was used in conjunction with Scherer's approach. The results of the calculations are given in Table 2. The smaller particle size of the TiO 2 may lead to a larger specific surface area and the surface-to-volume ratio of the solar cell, as well as an increased bandgap, which can increase the efficiency of the solar cell. Several investigations and measurements have led researchers to the conclusion that reducing the crystalline size of cells based on TiO 2 can help to improve their photovoltaic output by increasing electron lifetime, facilitating faster electron transport, increasing charge collection efficiency, and reducing the amount of recombination that occurs 27 . www.nature.com/scientificreports/ As can be seen in Fig. 3, the OH-stretching and bending vibrations are responsible for the absorption bands in the spectra, which were found at 3426 and 1620 cm −1 respectively. Additionally, between 500 and 900 cm −1 , a band of Ti-O was detected. The intensities of the OH bands (both stretching and bending) and the Ti-O bands, on the other hand, dropped as the amount of Cu in the sample increased 28 . The fact that the peaks in the area between 500 and 1000 cm −1 are distinct from those of pure CuO and pure TiO 2 implies the formation of new metal-oxygen bonds. These findings provide credence to the hypothesis that mixed oxide (Ti-O-Cu) bonding was formed since evidence of this kind of bonding was seen at 2922 cm −1 . After being treated, it was found that the TiO 2 photocatalyst had a significant quantity of water vapor and surface hydroxyl groups adsorbed to it 29 .
Microstructure analysis. SEM micrographs of the Cu composite reinforced with nano-TiO 2 were illustrated in Fig. 4. Figure 4a and b show the pure Cu, and pure TiO 2 , respectively. The microstructure of Cu-TiO 2 composite powder, in Fig. 4c-f are corresponding to 10, 20, 30, and 40 wt.% TiO 2 in the Cu matrix, respectively. As known, the necessary parameter in the production of nanocomposites is proper and well-dispersal nano reinforcement in the metal matrix. The nano-TiO 2 particles were distributed in an even manner throughout the Cu matrix composite, as shown in Fig. 4. A further observation that can be made is that the nano-TiO 2 reinforcing material was dispersed appropriately, entrapped inside the Cu matrix, and adhered very strongly since TiO 2 is a substance that is useful for the operation of solar cells. As a result, enhancing the photocatalytic activity of Additionally, TiO 2 is a ceramic substance that may function as an internal pore, so reducing the particle size of the copper. This results in an increase in surface area, which in turn results in an increase in photocatalytic activity. The as-resaved TiO 2 raw powder amounts in both the anatase and rutile phases and rutile had very minor peaks, which only showed in the high% of TiO 2 (40%). These findings indicate that the 40% sample contains two  www.nature.com/scientificreports/ different types of tetragonal TiO 2 . However, when the ratio of TiO 2 is smaller, the rutile peaks do not stand out as clearly because of the low ratio, which causes the peak intensity to be rather low.
Because of the effective milling procedure, Cu and TiO 2 particles have been found to have achieved a high level of homogeneity across all the samples. When the ball-to-powder ratio is optimized at 20:1, the milling period is extended to 24 h, and the rotating speed is increased to 250 revolutions per minute (rpm), copper particles undergo strain hardening and fracture, which results in a decrease in particle size. The grain size of Cu particles has been reduced following the rise in the percentage of TiO 2 present. This may be because of the ceramic quality of the TiO 2 particles; these particles function as internal balls and cause fractures in the Cu particles. For Fig. 4c-f, TiO 2 particles are well embedded in the Cu particles during the mechanical milling process. Also, they are distributed in the Cu matrix in a good manner. TiO 2 in nano 50 nm and A Copper powder (supplied by Alpha Chemicals, USA) with a 10 μm. So, the small particles in the SEM images correspond to TiO 2 and the larger particles belong to copper.
Few TiO 2 particles are aggregates as pockets, for 40wt.% TiO 2 samples. This may be attributed to the large surface area between the metallic Cu particles and the ceramic TiO 2 ones. There is no wettability between them. Also, a high difference between their melting points. The EDS analysis of Cu-TiO 2 samples is shown in Fig. 5 and Table 3 as the composite powders contain peaks for Cu, Ti, and O atoms. And all the prepared composite powders do have not equiaxed grains.

Identification of the bandgap energy.
The UV-vis-IR spectra are shown in Fig. 6, and they indicate how the reflectance of the UV-vis-IR spectrum is affected by the different concentrations of produced copper dopant. As the concentration of TiO 2 increased, it was discovered that the reflectance shifted into the lightvisible zone, and this was caused by the shift of the bandgap energy is shown to be lower when there is a higher concentration of TiO 2 . According to the Kubelka-Munk theory, the Schuster-Kubelka-Munk function is given in terms of the optical bandgap (Eg) as:  The value of the exponent n signifies the nature of the transition, with n = 1/2 or 2 for the direct/indirect allowed transition, respectively. Therefore, the bandgap energy may be evaluated from the reflectance spectra by extrapolating the straight-line plot of (F(R ∞ ) *hν) 2 or (F(R ∞ )*hν) 1/2 versus (hν) as shown in Fig. 7. Table 4 shows the values of the bandgap for different TiO 2 concentrations.  www.nature.com/scientificreports/ This change in the bandgap can be likely due to the fusion of Ti ions into the Cu crystal structure, and the defect centers formed by the substitution of Cu by Ti ions in the Cu crystal lattice resulting in changes in the optical absorption. The band gap can be determined also by the following formula: where h (Planks constant) = 6.63 × 10 -34 J.s; C (speed of light) = 3.0 × 10 8 m/s; λ cutoff (cut off wavelength) = 4.11 × 10 -7 m. Note: 1 eV = 1.6 × 10 -19 J (the conversion factor).
The appropriate concentration of TiO 2 in the Cu matrix is from 20 to 40% for solar cell application. This is because of the necessity of reinforced material in the improvement of light-harvesting of the cell, and the important characteristics of TiO 2 mesoporous materials. These characteristics like high specific surface area, pore size distribution and providing more reactive sites at surfaces for photocatalytic reactions. TiO 2 antireflection coatings in solar cell. According to the excellent optical properties and low deposition cost of the Titanium dioxide (TiO 2 ) thin films, they have a long history in silicon photovoltaics (PV) as antireflection (AR) coatings. This study identifies several unexplored applications for Cu-TiO 2 thin films, including the enhancement of silicon (Si) solar cell performance, the reduction of costs associated with device manufacture, and the simplification of the preparation process 30 . A technology known as chemical vapor deposition (CVD) was used to deposit Cu-TiO 2 layers. The facility is provided by the Nano lab at ERI, which aids the team. A single-layer antireflection coating, abbreviated as SLAR, is the bare minimum required for silicon solar cell manufacturing in today's world. Silicon and other materials that are semiconductors may be used effectively to absorb light. On the other hand, these substances have relatively high refractive indices 31 .
The variety of doping concentrations and shows the spectrum distribution of the optical transmittance of copper doped TiO 2 films. This figure also demonstrates the range of doping concentrations. The portion of the electromagnetic spectrum that is ultraviolet as well as the visible part were employed to carry out the test that investigated the coated films' level of transmittance. In addition, the optical transmittance values drop when there is a higher concentration of copper. This behavior is brought on by an increase in the number of electrons that are set free whenever there is a greater concentration of copper present in the system.
The refractive index of silicon is n si = 3.939 at 600 nm. This refractive index is much greater than air, which has a constant refractive index of n 0 = 1.0, and glass (n 0 = 1.52 at 600 nm). The reflectance of normally incident light at such an interface is given by: which means that in the first bounce, about 35.4% or 19.6% of the light is reflected off an air: silicon or glass: silicon interface, respectively. If an optimum-thickness AR coating is inserted between the silicon and ambient medium, the minimum reflectance is given by: where n AR is the coating refractive index. To achieve zero reflectance at one wavelength, the value of n AR should be. and the film thickness (d AR ) must meet the quarter-wave optical thickness requirement which can be formulated as: The formula is related to the design of antireflection coatings for optical surfaces. Here are the relevant bases:d AR represents the thickness of the antireflection coating in nanometers (nm), λ 0 represents the wavelength of the incident light in vacuum, typically in units of nanometers (nm), n AR represents the refractive index of the antireflection coating at the wavelength λ 0 .
The formula is derived from the principle of optical interference. When light is incident on a thin film with a thickness d and refractive index n, some of the light is reflected at the air-film interface, and some of it is transmitted through the film. The reflected and transmitted light waves interfere with each other, and the resulting interference pattern determines the amount of reflected light. For an antireflection coating, the goal is to minimize the amount of reflected light at a specific wavelength λ 0 . This can be achieved by choosing a thickness d AR and refractive index n AR such that the reflected light waves interfere destructively, cancelling each other out. The formula d AR = λ 0 /(4n AR ) gives the optimal thickness of the antireflection coating for achieving this interference pattern at the wavelength λ 0 .
There are many parameters for choosing the antireflection material like resist corrosion, withstand high temperatures, and other many parameters. Cu reinforced with TiO 2 can be used as the optimum material for SLAR. Controlling the ratio of TiO 2 in copper can achieve the required film thickness and reflectivity. Due to Eqs. (5) and (6), The AR coating should have 1.98 and 75.6 nm for refractive index and thickness, respectively. These (4) E g = hC = 1240/ cutoff , R = ( n 2 AR − n 0 n si n 2 AR + n 0 n si ) 2 , (7) n AR = √ n 0 n si ,  Figure 8 showed the Cu-TiO 2 as an AR coating for a silicon solar cell. It can be achieved under the condition of a fixed reflection index at the visible region 32 .
Electrical and thermal conductivities. A solar cell is an electrical device that converts light energy directly by the photovoltaic effect. It is a type of photoelectric cell. So, it has electrical characteristics, like current, electrical resistance, or voltage which vary when exposed to light. Solar cell considers electrical building blocks of photovoltaic modules, called solar panels. Electrons are excited from their orbital. It can dissipate the energy as heat and returns to its orbital. Current flows through the material to cancel the potential and this electricity is captured. So, studying the electrical and thermal conductivity is a good indication of the quality of the solar cell. Figure 9 shows the effect of TiO 2 additions on the electrical conductivity of Cu-TiO 2 nanocomposite powders. It decreases gradually by increasing the TiO 2 %. This can be attributed to the lower electrical conductivity of TiO 2 than that of Cu. As the electrical resistivity of TiO 2 is 420 nΩ.m, while that of Cu is 16.78 nΩ.m. So, TiO 2 resists the following of electronic charge carriers more than Cu. Consequently, the electrical conductivity decreases 33 . Figure 10 displays the relation between the TiO 2 % and the thermal conductivity of the Cu matrix. It is decreased gradually by increasing the TiO 2 %. This can be explained by the lower thermal conductivity value of TiO 2 than that of Cu, which, is 21.9 W/m.K for TiO 2 & 401 W/m.K for Cu. So, according to the rule of mixture the overall thermal conductivity of Cu-TiO 2 , nanocomposites is decreased by the addition of lower conductivity TiO 2 particles. In which TiO 2 restricts the heat transfer in the Cu matrix.
It must be noted that, although TiO 2 additions to the copper matrix, decreases both the electrical and thermal conductivities, it is still in the working area of Cu applications. As reinforcing Cu with TiO 2 did not convert copper into non-conductive material, only decreases its conductivity.

Conclusions
In this paper, Cu-TiO 2 nanocomposite powders have been successfully prepared by the mechanical milling method. In this preparation method, the weighted composite powders were mixed in a stainless-steel vial and protected from oxidation using pure argon, by a 20:1 steel ball-to-powder ratio (BPR), a ball diameter of 5 mm, and a rotational speed of 250 rpm. Various content of nano-TiO 2 particles successfully reinforced the Cu matrix composite and uniformly distributed inside the matrix through the fabrication process of the powder metallurgy technique. Cu-TiO 2 has been characterized by using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Scanning Electron Microscope (SEM) to determine their crystal structure, and UV-visible absorption spectrometry (UV-Vis) to estimate the optical properties. The X-ray diffraction pattern showed peaks corresponding to Cu and TiO 2 . There was no record of any other interfering intermetallic compounds in the XRD pattern. On the other hand, SEM images showed a proper and homogenous dispersal of TiO 2 in the fabricated composite matrix. This paper also studied the influence of various prepared TiO 2 dopant concentrations on the UV-vis-IR reflectance. It can be observed that increasing TiO 2 concentration, increases the reflection % which is good in different applications related to solar cell fabrication.

Data availability
All data generated or analyzed during this study are included in this published article.